threejs get center of object

The object boundingBox is in local space. If you want the center in world space you have to translate your middle vertex to world space. You can do that easily with the THREE.Object3D localToWorld method like this:

function getCenterPoint(mesh) {
    var middle = new THREE.Vector3();
    var geometry = mesh.geometry;

    geometry.computeBoundingBox();

    middle.x = (geometry.boundingBox.max.x + geometry.boundingBox.min.x) / 2;
    middle.y = (geometry.boundingBox.max.y + geometry.boundingBox.min.y) / 2;
    middle.z = (geometry.boundingBox.max.z + geometry.boundingBox.min.z) / 2;

    mesh.localToWorld( middle );
    return middle;
}

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Update:

In the newer three.js versions there is a convenience method getCenter inside the THREE.Box3 class. Since the bounding box is in local coordinate space you will need to project the result to worldspace to get the center in world coordinates, but this is easy with the localToWorld method from the parent object.

function getCenterPoint(mesh) {
    var geometry = mesh.geometry;
    geometry.computeBoundingBox();
    var center = new THREE.Vector3();
    geometry.boundingBox.getCenter( center );
    mesh.localToWorld( center );
    return center;
}

try the code below:

let box = new THREE.Box3().setFromObject(myObject3D)
let sphere = box.getBoundingSphere()
let centerPoint = sphere.center

THREE.Box3().setFromObject(Object3D) will traverse the Object3D's geometries, and return their holistic bounding Box.